Due to their processability and useful mechanical properties, thermoplastic polymers have found widespread commercial success in consumer goods, military applications, packaging materials, construction, automobiles, and electronics. However, the vast majority of plastics are electrical insulators, which can present problems with static (triboelectric) charge build-up. Electric charge that builds up, triboelectrically or otherwise, on the surface of an insulating plastic may not easily move across the surface of the article to recombine. Applications where static build-up on plastics can be problematic include aircraft, electronics, and military fabrics/devices. Polymers with electrostatic dissipation properties are used in a wide variety of applications, notably for the prevention of explosions due to charge build up in engines and turbines (e.g., U.S. Published Patent Application No. 2008/0237527 and U.S. Pat. No. 7,527,042) and for the protection of electronics (e.g., U.S. Pat. Nos. 5,916,486, 5,997,773, and 6,015,509, and 6,687,097; 6,344,412; 6,534,422; and 6,459,043).
Electronics can be damaged by static discharge, and combustible or energetic materials can be ignited or detonated, respectively. In consumer applications, plastic electrostatic dissipation materials can be useful in flooring (e.g. tiles or carpet) and automobile seat covers, which can otherwise build up a static charge due to friction against shoes or clothing. If electrical insulators are used, these materials can deliver unwanted “shocks” to the consumer, especially under dry atmospheric conditions. Although conducting polymers that make excellent electrostatic dissipation materials are now commercially available, their cost per kg is prohibitively higher than commodity plastics, their processability in common plastics processing equipment is poor, and their mechanical properties are not well-suited to many applications. Therefore, traditional commodity plastics are usually blended with anti-static agents or conducting fillers to achieve electrostatic dissipation properties.
Materials hereinafter referred to as having “electrostatic dissipation characteristics” have surface resistivity in the range of 1×105 to 1×1012 Ω/□ (or volume resistivity in the range of 1×104 to 1×1011 Ω-cm). Many unmodified plastic resins have surface resistivity in the range of 1×1014 to 1×1016 Ω/□, meaning they are electrical insulators. To lower the surface resistivity into the electrostatic dissipation range, a variety of processing strategies have been described.
U.S. Published Patent Application No. 2009/0186254 provides an acid-cured resole with no additional additives that possesses low enough resistivity for electrostatic dissipation applications. The synthesis of a poly(urea-urethane) based on poly(dimethylsiloxane), which exhibits sufficient conductivity at high humidity for electrostatic dissipation applications, is disclosed in U.S. Pat. No. 6,841,646.
Inclusion of moieties capable of conduction within the polymer backbone can also be a successful strategy for electrostatic dissipation materials. U.S. Pat. No. 6,586,041 describes a method for achieving electrostatic dissipation in a transparent polymer material, made from conductive polymer, crosslinkable polymer, and crosslinking agent, for use as a thin film for packaging materials. U.S. Pat. No. 7,041,374 details the use of metallocene moieties within a polymer backbone to confer electrostatic dissipation properties.
In another class of electrostatic dissipation polymers, a chemical additive can be either applied to the surface or incorporated beneath the surface of the article. It is common to introduce anti-static agents during processing, which are often amphiphilic surfactant molecules. The anti-static agent typically has an ionic “head” group and a long, non-polar “tail” group. Due to poor thermodynamics of mixing between the ionic head group and the plastic, the anti-static agent migrates to the surface after processing, forming a highly polar layer at the surface. Due to adsorption of atmospheric moisture onto the surface, the surface resistivity is significantly lowered.
The anti-static agent may be partially lost due to wear or friction on the surface during normal use of the plastic article, but the anti-static agent is self-replenishing to some extent because additional anti-static agent is able to diffuse to the surface over time. While traditional anti-static agents may be useful for common consumer articles, for demanding applications such as outdoor use, the anti-static agents can be lost due to weathering and/or mechanical abrasion. Therefore, approaches involving an immobilized anti-static agent are preferable. For example, U.S. Pat. No. 5,571,472 provides for achieving electrostatic dissipation properties in a shaped resin without damaging physical properties or causing discoloration, via addition of a nitrogen-containing compound during molding, followed with corona discharge treatment to the surface of the shaped article.
Fillers, such as metal particles or carbon fibers, can also be added to a polymer article in order to achieve sufficient conductivity for electrostatic dissipation. Carbon black has been demonstrated for this purpose, as have carbon nanotubes and metal nanocomposites. U.S. Published Patent Application No. 2009/0236132 discusses electrostatic dissipation materials comprised of a dispersion of non-insulating particles, such as gold or carbon black, and an inherently dissipative polymer within a thermosetting resin. U.S. Published Patent Application No. 2009/0281227 discloses a polymer composition consisting of a poly(aryl ether ketone), a poly(biphenyl ether sulfone), and a fibrous carbon nanofiller that possess electrostatic dissipation properties.
U.S. Pat. No. 7,236,396 and U.S. Published Patent Application No. 2004/0126521 describe a high temperature, high strength polymer which uses a metal oxide to achieve electrostatic dissipation for read/write heads in magnetic media. Additionally, these references describe the ability of the material to be dyed via a pigment while maintaining their conductive properties.
U.S. Pat. No. 7,476,339 details the use of non-carbonaceous fillers, such as metal oxide particles, within thermoplastic polymers, for electrostatic dissipation properties. U.S. Published Patent Application No. 2005/0194572 discusses the polymerization of a thermoplastic in the presence of a lithium salt, resulting in electrostatic dissipation capability.
Graphite-filled polymer composites for electrostatic dissipation applications are described in U.S. Pat No. 6,746,626. U.S. Published Patent Application No. 2006/0047052 and U.S. Pat. No. 7,897,248 disclose a method for orienting nanotubes within a polymer matrix, leading to an electrostatic dissipation-capable material. A degradable polymer can be used, along with metal flakes, fibers or powders, to achieve electrostatic dissipation properties in a moldable article, as described in U.S. Pat. No. 5,904,980.
Ion-conducting polymers potentially represent another means of lowering the resistivity of a polymeric article. Of particular relevance to the present disclosure are polyamine hydrohalides, a type of ion-conducting polymer. Electrical properties of poly(2-dimethylaminoethylmethacerylate) and its hydrochloride salt, an ion-conducting polymer, have been recently reported (Chetia J R, MauHick M, Dutta A, Dass N N. Materials Science and Engineering B-Solid State Materials for Advanced Technology 2004; 107(2):134-138). The electrical conductivity of the ion-conducting polymer was found to be in the range of 10−2 to 10−4 S cm−1. Incorporation of an ion-conducting polymer into a polymeric matrix is therefore another approach to lowering the electrical resistivity of the material.
Also relevant to the present disclosure is the infusion processing of plastic materials. The optical, mechanical, and electrical properties of various plastics may be enhanced by infusion of compounds, functional additives, or monomers from solution. Infusion of coloring agents and functional additives into polymeric matrices and to articles comprising such matrices has been disclosed in U.S. Pat. Nos. 6,749,646; 6,929,666; 7,094,263; 6,733,543: 6,949,127; 6,994,735; and 7,175,675.